1. Field of the Invention
This invention relates to electrical apparatus.
More particularly the invention relates to electrical apparatus of the kind (hereinafter referred to as the kind specified) comprising a number of units, each of which is capable of presenting to an input signal a transfer function of a different value and control means whereby a desired selection of said units can be made so as to present to the input signal a total value of desired transfer function value within a predetermined range.
2. Description of the Related Art
In such an apparatus the control means typically provides a binary control signal, each digit of which determines whether or not a different one of the unit is selected according to the value of that digit i.e. whether that digit has a value `0` or `1`.
The control signal is suitably produced by an analogue to digital convertor which produces a binary digital output signal in response to an analogue input signal representing the desired total value of said transfer function to a resolution, i.e. number of digits, equal to the number of said units.
One example of such an apparatus is a phase shifter which comprises a number of units each of which imposes a different phase shift on the input signal. In such an apparatus the values of the transfer functions presented by the different units are chosen so that any desired value within the given range of values of the transfer function can be obtained with at least a predetermined accuracy i.e. resolution. To this end in a known form of apparatus of the kind specified the values of the transfer functions presented by the units are chosen so that the available settable total values of the transfer functions are equally spaced through the desired range. In such an arrangement the unit presenting the largest value of transfer function presents a transfer function equal to half the desired range, a second unit presents a transfer function of half this value and so on, the unit presenting the smallest value of transfer function presenting a value equal to or less than the desired resolution. For example for a phase shifter settable in the range 0.degree. to 360.degree. using five units and have a five digit binary number as the control signal the phase shifts of the different units are 180.degree., 90.degree., 45.degree., 22.5.degree. and 11.25.degree.. With such an arrangement any desired value within the range 0.degree. to 360.degree. can be obtained with a maximum resolution of 11.25.degree.. However, in practice the actual phase shifts presented by the different units are likely also to be subject to error. As a result the actual maximum resolution is larger than 11.25.degree.. For example if each unit is subject to the same percentage error and the unit giving the largest phase shift is high and the other units are all low a large gap in obtainable values exists between that obtained when all but the highest phase shift unit is selected (i.e. control signal 01111) and that obtained which only the unit giving the largest phase shift (MSB) is selected (i.e. control signal 10000).